1. Field of the Invention
The present invention relates to an on-vehicle radar mounted on a vehicle for detecting a preceding vehicle or the like.
2. Description of the Related Art
A system shown in FIG. 4 is known as this kind of on-vehicle radar. In FIG. 4, the known on-vehicle radar comprises a transmitting/receiving means 6 including an antenna 1 for transmitting and receiving a radio wave, a coupler 2, a voltage control oscillator 3, a frequency converter 4, and a gain control unit 5, an arithmetic control unit 9 including a modulation signal control unit 7, a frequency analyzer 8, and a signal processor 10 for transferring signals to or from the transmitting/receiving means 6, and a transmission/reception direction control means in the form of a machine drive unit 11 for controlling the transmission and reception direction of a radio wave which is transmitted and received by the transmitting/receiving means 6 in response to a command issued from the arithmetic control unit 9. The machine drive unit 11 mechanically drives and controls the transmitting/receiving means 6 so as to change the transmission and reception direction in which the antenna 1 transmits and receives a radio wave in response to a command issued from the arithmetic control unit 9.
Next, the operation of the known on-vehicle radar having the above-mentioned components will be described. The modulation signal control unit 7 supplies a modulation signal instructing the voltage control oscillator 3 to generate a radio wave with a relatively high frequency which is linearly frequency-modulated. The radio wave with a relatively high frequency which is linearly frequency-modulated is radiated from the antenna 1 to the air via the coupler 2. A received wave emanating from an object that reflects a transmitted wave is caught through the antenna 1 and supplied to the frequency converter 4.
The frequency converter 4 mixes part of a transmitted radio wave sent from the coupler 2 with a received radio wave sent from the antenna 1, and generates a signal with a relatively low frequency. The frequency of a received radio wave has undergone a frequency shift dependent on a delay time of a radio wave proportional to a distance to an object. When an object is moving, the frequency of a received radio wave has undergone a Doppler shift dependent on the velocity of the moving object. The signal with a relatively low frequency sent from the frequency converter 4 is therefore a multivalued signal (video signal) containing information such as a relative distance to an object and a relative speed of the object. Every time the machine drive unit 11 scans a transmitted and received radio wave, the gain control unit 5 controls the power of the multivalued signal so that the power will be set to a proper level. The arithmetic control unit 9 calculates the relative distance and relative velocity using frequency data sent from the frequency analyzer 8.
Next, a method of calculating a relative distance and relative velocity will be described. FIG. 5 shows an example of calculating the relative distance and relative velocity using the on-vehicle radar. In FIG. 5, reference numerals have the following meanings:
C: light velocity=3.0.times.10.sup.8 m/sec PA1 .lambda.: wavelength of a transmitted radio wave (for example, if the reference frequency F0 of a transmitted radio wave equals to 60 GHz, .lambda. indicates 5.0.times.10.sup.-3 m) PA1 B: frequency scanning bandwidth PA1 Delay time in relation to a relative distance R: Td=2R/C PA1 Doppler shift in relation to a relative velocity V: Fd=2V/.lambda. PA1 Fbu: frequency difference between a transmitted signal and a received signal occurring when the frequency of the transmitted signal is raised PA1 Fbd: frequency difference between a transmitted signal and a received signal occurring when the frequency of the transmitted signal is lowered
When an object reflecting a radio wave exhibits no relative velocity, since the frequency differences Fbu and Fbd have the relationship of: EQU Fbu=Fbd=(B/(Tm/2)).times.(2R/C)=4.times.B.times.R/Tm.times.C
the relative distance to the object is expressed as follows: EQU R=(Tm.times.C/4.times.B).times.Fbu (=Fbd) (1)
Moreover, when an object reflecting a radio wave exhibits a relative velocity, since the frequency differences Fbu and Fbd are given by: EQU Fbu=(B/(Tm/2)).times.(2R/C)-2V/.lambda.=4.times.B.times.R/Tm.times.C-2V/.la mbda. EQU Fbd=(B/(Tm/2)).times.(2R/c)+2V/.lambda.=4.times.B.times.R/Tm.times.C+2V/.la mbda.
the relative distance to the object and the relative velocity of the object are expressed as follows: EQU R=(Tm.times.C/8.times.B).times.(Fbu+Fbd) (2) EQU V=(.lambda./4).times.(Fbd-Fbu) (3)
Moreover, assuming that the resolution of a frequency .DELTA.F (=1/(Tm/2)) is equal to 4.times.B.times.R/Tm.times.C or 2V/.lambda., that is, EQU .DELTA.F(=1/(Tm/2))=4.times.B.times..lambda.R/Tm.times.C
the resolution of the relative distance, .DELTA.R, is expressed as follows: EQU .DELTA.R=C/(2.times.B) (4)
Since the resolution of a frequency, .DELTA.F, is expressed using the equation (4) as follows: EQU .DELTA.F(=1/(Tm/2))=2.DELTA.V/.lambda.
the resolution of the relative velocity, .DELTA.V, is given by the following equation: EQU .DELTA.V=.lambda./Tm (5)
For example, when EQU B=C/(2.times..DELTA.R)=3.0.times.10.sup.8 /(2.times.0.5)=3.0.times.10.sup.8
if .DELTA.R should equal to 0.5 m, B is 300 MHz. Moreover, when EQU Tm=.lambda./.DELTA.V=5.0.times.10.sup.-3 /(1/3.6)=18.times.10.sup.-3
if .DELTA.V should equal to 1 km/h, Tm is 18 msec.
The aforesaid known on-vehicle radar is adapted to, for example, an inter-vehicle distance alarm system for informing a driver of danger by sounding an alarm when an inter-vehicle distance relative to a preceding vehicle falls below a safe inter-vehicle distance and the risk of a collision gets higher, or to an inter-vehicle distance control system for tracking a preceding vehicle with a safe inter-vehicle distance retained.
Some released literatures describe a technique for improving the resolution of a distance by changing a modulation cycle or a technique for simplifying pairing needed for identifying the same target. That is to say, Japanese Unexamined Patent Publication No. 8-136647 (Honda Motor Co., Ltd.) has disclosed a technique for improving the resolution of a distance by shortening a modulation cycle (time) so as to decrease a normalized value of a quotient of a distance by a beat frequency. Japanese Unexamined Patent Publication No. 8-189965 (Honda Motor Co., Ltd.) has disclosed a technique for improving the resolution of a distance, wherein when a target is running at a high speed, a modulation cycle (time) is extended in order to widen a range of sensed distances; and when a target is running at a short distance, the modulation cycle (time) is shortened in order to confine the range of sensed distances to short distances. Japanese Unexamined Patent Publication No. 8-211145 (Toyota Motor Co.) has disclosed a technique for simplifying pairing needed for identifying the same target by selecting signals with the same amplitude, wherein when a modulation cycle (time) is shortened, the resolution of a velocity decreases, but a difference in amplitude of a signal reflected from a target from an original signal frequency-modulated to rise or fall gets smaller.
In general, in an on-vehicle radar, a radar equation below is established. The maximum sensed distance of the radar is proportional to the sensitivity or minimum required input of the radar (1/4 squared). EQU R0.sup.4 =R.sup.4 .times.10.sup.(0,2).alpha.,R ={Pt.times.Gt.times.Gr.times..lambda..sup.2 .times..sigma.}/{(4 .pi.).sup.3 .times.Smin.times.(S/N).times.LSYS.times.LAGC}(6)
where R0 is an ideal maximum sensed distance with no consideration taken into an atmospheric damping factor, R is an actual maximum sensed distance, .alpha. is an atmospheric damping factor, Pt is a transmitted power level, Gt is an antenna gain for transmission, Gr is an antenna gain for reception, .lambda. is the wavelength of a transmitted radio wave, .sigma. is an effective area of an object capable of reflecting a radio wave, Smin is the sensitivity or minimum required input of the radar, S/N is a sensing coefficient, LSYS is a loss occurring in the radar system, and LAGC is a decay to be applied by the gain control unit. When an antenna used in common for transmission and reception is employed, Gt equals to Gr.
In the aforesaid on-vehicle radar, generally, when a sensed distance ranges from 3 m to 160 m, a change in power of a radio wave received by the on-vehicle radar (dynamic range) is expressed as the following equation (7): EQU 40 log (160 m/3 m).apprxeq.69 dB (7)
However, since the aforesaid on-vehicle radar is used while mounted on a vehicle, the environment is noisy. Under the circumstances, an A/D converter (analog signal-to-digital signal converter) employed in the arithmetic control unit 9 of the on-vehicle radar must produce a signal exhibiting a specified signal-to-noise ratio. The produced signal is therefore 8 bits long or 12 bits long at most. Assuming that one of all the bits is assigned to a plus or minus sign and two bits thereof are used to monitor a noise level, a change in power (dynamic range) that can be indicated by the remaining bits is as mentioned below.
When the signal produced by the A/D converter is 8 bits long, the change in power that can be indicated is calculated as follows: EQU 20 log (2.sup.(8-3)).apprxeq.30 dB (8)
When the signal produced by the A/D converter is 12 bits long, the change in power that can be indicated is calculated as follows: EQU 20 log (2.sup.(12-3)).apprxeq.54 dB (9)
A change in power proportional to a range of distances sensed by the on-vehicle radar generally exceeds a change in power that can be handled by the A/D converter over the same time. This means that all objects existing in the range of sensed distances cannot be sensed.
A range of distances (RAD) within which the A/D converter can handle a change in power over the same time is calculated as described below.
When the signal produced by the A/D converter is 8 bits long, assuming that the maximum sensed distance is 160 m and 30 dB.apprxeq.40 log(160/Rmin) is established, the minimum sensed distance (Rmin) is approximately 28 m. Consequently, EQU RAD=28 m to 160 m (10)
Assuming that the minimum sensed distance is 3 m and 30 dB.apprxeq.40 log(Rmax/3) is established, the maximum sensed distance (Rmax) is approximately 17 m. Consequently, EQU RAD=3 m to 17 m (11)
When the signal produced by the A/D converter is 12 bits long, assuming that the maximum sensed distance is 160 m and 54 dB.apprxeq.40 log(160/Rmin) is established, the minimum sensed distance (Rmin) is approximately 7 m. Consequently, EQU RAD=7 m to 160 m (12)
Assuming that the minimum sensed distance is 3 m and 54 dB.apprxeq.40 log(Rmax/3) is established, the maximum sensed distance (Rmax) is approximately 67 m. Consequently, EQU RAD=3 m to 67 m (13)
In general, the gain control unit 5 applies a delay (LAGC) according to the magnitude of power of a received radio wave so that the A/D converter employed in the arithmetic control unit 9 will not be saturated. Thus, the sensitivity (Smin) or minimum required input is degraded apparently in order to shorten the maximum sensed distance (Rmax). An object is sensed in units of the range of distances (RAD) from the maximum sensed distance (Rmax) to the minimum sensed distance (Rmin) permitting the A/D converter to remain unsaturated.
For example, when an object running at a relatively short distance of 3 m on the same lane as an own vehicle is sensed, since the power of a received radio wave is high, every time the machine drive unit 11 scans one transmitted and received radio wave, the gain control unit 5 applies a decay (LAGC) to the received radio wave for the purpose of preventing the A/D converter from being saturated. Thus, the power of the received radio wave is set to a relatively low level. The arithmetic control unit 9 then calculates a relative distance and a relative velocity.
However, as a result, apparently, the sensitivity or minimum required input (Smin) is degraded and the noise level is raised. An object running at a relative long distance of 160 m cannot be sensed because the power of a received radio wave is so low as to become indistinguishable from the noise level.
When the radar is employed in an inter-vehicle distance alarm system or inter-vehicle distance control system, priority is given to sensing of a target running at a short distance on the same lane as an own vehicle. The above drawback poses therefore few serious problems. However, since roads are congested these days, there may be a vehicle running on an adjacent lane or a large object nearby. In this case, when a radio wave of a relatively high power level is received, every time the machine drive unit 11 scans a transmitted and received radio wave, if the gain control unit 5 responds immediately to the received radio wave and applies a decay (LAGC) to the radio wave, the sensitivity (Smin) or minimum required input is degraded apparently. This causes the noise level to rise. An object existent on the same lane as the own vehicle which has been sensed on the verge of the sensitivity or minimum required input will not be able to be sensed any longer.
Assuming that the on-vehicle radar is employed in an inter-vehicle distance alarm system or inter-vehicle distance control system, when an object existent on the same lane as an own vehicle is sensed, if there is a vehicle running on an adjacent lane or a large object in the vicinity of the own vehicle, the object running on the same lane will not be able to be sensed any longer. The aforesaid drawback poses a serious problem in practice.